1. Field of the Invention
The present invention relates to a fuel injection pipe used as a fuel supplying path of a diesel internal combustion engine and more particularly to a thick high-pressure fuel injection pipe, whose diameter is relatively thin, of about 6.0 mm to 15.0 mm in diameter and about 2.2 mm to 5.5 mm in thickness.
2. Description of Related Art
While a steel pipe in the class of 340N/mm.sup.2 to 410N/mm.sup.2 of tensile strength had been used conventionally for a fuel injection pipe of a diesel engine, a high internal pressure of 1200 bar or more, which had been the maximum in the past, has come to be loaded to the fuel injection pipe and it has come to be required to have a higher fatigue strength to sustain the high internal pressure because a method of injecting high-pressure atomized fuel has come to be adopted to burn the fuel within engine cylinders more completely to exhaust cleaner exhaust gas along the development of the exhaust gas purifying technology conducted due to the restriction on exhaust gas of diesel engine. As a countermeasure thereof, there is a tendency of using a high tension steel in the class of 490N/mm.sup.2 to 800N/mm.sup.2 of tensile strength. Such a high tensile steel pipe is manufactured by drawing in general.
However, the high tension steel pipe manufactured by drawing sometimes causes fine wrinkles of about 100 .mu.m in depth on the inner surface thereof in tubing the pipe from hot ingot and in subsequently working the tube into a pipe having necessary dimensions by drawing (elongation). The wrinkles have been known to be caused by a difference of flows of the material inside and outside of the tube which arises when the tube is worked by reducing by a die from the outside of the tube and by rolling by a plug from the inside thereof. That is, such phenomenon occurs due to the deficiency of elongation which is caused by that the tension and the elongation (ductility and workability) are almost inversely proportional. It occurs remarkably in a thick tube. Further, because it has less ductility, the wrinkles on the inner part which is rolled by the plug also remain as wrinkle flaws. Especially when fine wrinkles of about 100 .mu.m in depth exist on the inner surface of the pipe, there has been a danger of causing a fatigue destruction and of bursting the pipe because concentration of stress occurs at the wrinkled portion when a high internal pressure exceeding 1200 bar is repeatedly applied within the pipe.
Further, because drawing reduction is limited per each drawing of the high tension steel pipe, it becomes necessary to draw high tension steel pipe a greater number of times in order to obtain a necessary dimensions of the pipe as compared with lower tension steel pipe. Therefore, there has been a problem that the production cost of the fuel injection pipe is high because, beside the low productivity thereof, it is obliged to increase equipment costs by installing a tube elongating machine, a swaging machine, a correcting machine and the like.
For example, in case of the high tension steel pipe in the class of 800N/mm.sup.2 of tensile strength, a cross-sectional reduction applicable in one time of drawing is limited to about 30% in the current facility. Therefore, in case of a typical seamless pipe of mother pipe size of .phi.34 mm, while it has been possible to finish it to a size of an injection pipe by four times of drawing (number of times of tube elongation) in the conventional class of 340N/mm.sup.2 to 410N/mm.sup.2 of tensile strength, it is necessary to draw eight times in case of the high tension steel pipe in the class of 800N/mm.sup.2 of tensile strength.
Further, in case of the high tension steel pipe in the class of 800N/mm.sup.2 of tensile strength, there has been a problem that various limits related to drawing of the high strength member such as the seizure of a tool such as a die, the burden placed on the swaging machine, the reduction of a number of pipes simultaneously drawn from the pipe elongating machine and the like must be solved, beside that it is obliged to increase the process.